spectrum of hexane
Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study
Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US
K12 grade 11 grade 12 organic chemistry courses involving molecular
spectroscopy analysing mass spectra of hexane
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Mass spectroscopy - spectra index
comparing infrared, mass, 1H NMR & 13C NMR
spectra of the structural alkane isomers of C6H14
see The molecular structure,
naming of alkanes
Interpreting the fragmentation pattern of the mass spectrum of hexane
[M]+ is the molecular ion peak (M) with an m/z of
86 corresponding to [C6H14]+, the original hexane molecule minus an electron,
The small M+1 peak at m/z 87, corresponds to an ionised
molecule with one 13C atom in it i.e. an ionised hexane molecule of
Carbon-13 only accounts for ~1% of all carbon atoms
(12C ~99%), but the more carbon atoms in the molecule,
the greater the probability of observing this 13C M+1
Hexane has 6 carbon atoms, so on
average, ~1 in 17 molecules of will contain a 13C atom.
A similar argument applies to fragment ions from the
breakdown of the parent molecular ion of hexane - though the ratio
will be greater e.g. the m/z 58 ion.
Identifying the species giving the most prominent peaks
(apart from M) in the fragmentation pattern of hexane.
The most abundant ion of the molecule under mass
spectrometry investigation is usually given an arbitrary abundance value of
100, called the base ion peak, and all other abundances
('intensities') are measured against it.
|m/z value of
|m/z value of
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of hexane
Atomic masses: H = 1; C = 12 (13 for ~1
Bond enthalpies = kJ/mol: C-C = 348;
C-H = 412
Examples of possible equations to explain some of the most abundant ion peaks
in the mass spectrum of
Formation of m/z 71 ion:
[CH3CH2CH2CH2CH2CH3]+ ===> [CH3CH2CH2CH2CH2]+
C-C bond scission in the carbon chain of the
molecular ion of hexane giving a [C5H11]+
Mass change: 86 - 15 = 71.
Formation of m/z 57 ion:
[CH3CH2CH2CH2CH2CH3]+ ===> [CH3CH2CH2CH2]+
The m/z 57 ion is the base peak ion, the most
abundant and 'stable' ion fragment.
Again, C-C bond scission in the carbon chain of the
molecular ion of hexane.
Loss of ethyl group, mass change = 86 - 29 = 57.
Similarly the m/z 57 ion could be formed by further
scission of a shorter fragment ion.
e.g.: [CH3CH2CH2CH2CH2]+ ===> [CH3CH2CH2CH2]+
Formation of m/z 43 ion:
[CH3CH2CH2CH2CH2CH3]+ ===> [CH3CH2CH2]+
C-C bond scission of the parent molecular ion, loss
of propyl group, mass change = 86 - 43 = 43.
Formation of m/z 29 ion:
[CH3CH2CH2CH2CH2CH3]+ ===> [CH3CH2]+
C-C bond scission of the parent molecular ion, loss
of butyl group, mass change = 86 - 57 = 29.
Sequences including m/z values of 42, 41, 40, 39 or
28, 28, 27, 26, indicate
successive hydrogen atom/molecule loss from the m/z 43 or 29 ions.
Comparing the infrared, mass, 1H NMR and 13C NMR
spectra of the five structural alkane isomers of C6H14
NOTE: The images are linked to their
original detailed spectral analysis pages AND can be doubled in
size with touch screens to
increase the definition to the original hexane,
2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and
2,3-dimethylbutane image sizes. These five molecules
are structural isomers of saturated alkanes of molecular formula C6H14
exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower
aliphatic alkanes (non-cyclic alkanes).
Infrared spectra below.
Apart from the significant differences in the fingerprint region at
wavenumbers 1500 to 400 cm-1, there are no other
great striking differences, but each could be identified from
its infrared spectrum.
All the absorption
bands are typical of molecules containing saturated alkyl structure and
there are no characteristic infrared absorptions due to a specific
Infrared spectra above, mass spectra below.
MASS SPECTRA: Base ion
peaks plus m/z comments.
Hexane: m/z 57, 42 and 56 prominent
2-methylpentane: m/z 43, 42 and 71 prominent
3-methylpentane: m/z 57, 41 and 56 prominent
2,2-dimethylbutane: m/z 43, 41, 57 and 71
2,3-dimethylbutane: m/z 43, 41, 42 and 71
Mass spectra above, 1H NMR spectra below.
1H NMR SPECTRA: They can
all be distinguished by their different integrated proton ratios -
need very high resolution.
δ shifts, H ratio 3:2:2 (6:4:4 in formula)
δ shifts, H ratio 6:3:2:2:1
δ shifts, H ratio 6:4:3:1
2,2-dimethylbutane: 3 1H
δ shifts, H ratio 9:3:2
2,3-dimethylbutane: 2 1H
δ shifts, H ratio 6:1 (12:2 in formula)
1H NMR spectra above, 13C NMR spectra below.
13C NMR SPECTRA: From the
number of shifts, you can't distinguish (iii) and (iv) but you can
distinguish them from (i), (ii) and (v). (i) Hexane: 3 13C
(ii) 2-methylpentane: 5 13C
(iii) 3-methylpentane: 4 13C
(iv) 2,2-dimethylbutane: 4 13C
(v) 2,3-dimethylbutane: 2 13C
13C NMR spectra above.
Key words & phrases: image diagram on how to interpret and explain the mass spectrum of
hexane m/z m/e base peaks, image and diagram of the mass spectrum of
hexane, details of the mass spectroscopy of hexane, low and high resolution mass
spectrum of hexane, prominent m/z peaks in the mass spectrum of hexane, comparative
mass spectra of hexane, the molecular ion peak in the mass spectrum of hexane,
analysing and understanding the fragmentation pattern of the mass spectrum
of hexane, characteristic pattern of peaks in the mass spectrum of hexane, relative
abundance of mass ion peaks in the mass spectrum of hexane, revising the mass
spectrum of hexane, revision of mass spectroscopy of hexane, most abundant ions in the
mass spectrum of hexane, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of hexane, how to analyse the mass
spectrum of hexane, how to describe explain the formation of fragmented ions in the
mass spectra of hexane equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of hexane recognising the base ion
peak of hexane interpreting interpretation the mass
spectrum of hexane Stick diagram of the relative abundance
of ionised fragments in the fingerprint pattern of the mass spectrum of
hexane. Table of the m/e m/z values and formula of the ionised fragments in the
mass spectrum of hexane. The m/e m/z value of the molecular ion peak in the
mass spectrum of hexane. The m/e m/z value of the base ion peak in the
mass spectrum of hexane. Possible examples of equations showing the formation
of the ionised fragments in hexane. Revision notes on the mass spectrum of
Matching and deducing the structure of the hexane molecule from its mass
spectrum. Mass spectroscopy of
mass spectra of hexane, an isomer of molecular formula C6H14
The chemistry of ALKANES
revision notes INDEX
The infrared spectrum of hexane
The H-1 NMR spectrum of hexane
The C-13 NMR spectrum of hexane
Mass spectroscopy index
ALL SPECTROSCOPY INDEXES
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